SECOND MESSENGERS REGULATE INTRACELLULAR CA2+ (CA2+I) LEVELS THROUGH THEIR REGULATORY MECHANISMS ON INTRACELLULAR CA2+ STORAGE AND THROUGH THEIR MODULATORY EFFECTS ON VOLTAGE AND LIGAND-GATED CA2+ CHANNELS. CA2+ HOMEOSTASIS, AN ESSENTIAL COMPONENT OF INTRACELLULAR SIGNALLING, IS ALSO MAINTAINED BY ACTIVITIES OF CA2+ ATPASE, NA+-CA2+ - EXCHANGER AND INTRACELLULAR PROTEIN BUFFERS. WE EXAMINED STEADY-STATE CA2+ EFFLUX THROUGH THE PLASMA MEMBRANE OF APLYSIA BAG CELLS IN CULTURE USING THE CALCIUM-SENSITIVE VIBRATING PROBE TECHNIQUE AND ASSESSED THE EXTENT TO WHICH ANALOGS OF C-AMP, C-GMP, PHORBOL ESTER (TPA), CAFFEINE AND IP3 INFLUENCE THIS EFFLUX. BAG CELLS WERE CULTURED AS DESCRIBED BY KNOX ET AL., 1992. RECORDINGS WERE MADE AT LOW EXTERNAL CA2+ ~100 ~M. WHILE C-AMP AND PHORBOL ESTER REDUCED STEADY STATE CA2+ EFFLUX, C-GMP ENHANCED CA2+ EFFLUX THROUGH THE PLASMA MEMBRANE. CAFFEINE AND IP3 HAD NO EFFECT ON THE STEADY STATE CA2+ EFFLUX ON THE BAG CELLS. WE FURTHER EXAMINED THE EFFECTS OF THESE SECOND MESSENGERS ON VOLTAGE-GATED CA2+ CURRENTS UNDER VOLTAGE CLAMP CONDITIONS. WHEREAS C-AMP AND TPA ENHANCED THE VOLTAGE-GATED CA2+ CURRENT, C-GMP CAUSED A REDUCTION OF THE CURRENT. THE RESULTS SUGGEST THAT THESE SECOND MESSENGERS MAY HAVE A DIRECT MODULATORY EFFECT ON EITHER CA2+-ATPASE OR NA+-CA2+ EXCHANGER. HOWEVER, INDIRECT ACTION OF A SECOND MESSENGER BY EITHER RAISING OR REDUCING CA2+I AND THE ENHANCEMENT OR REDUCTION OF THE ACTIVITY OF THE CA2+-ATPASE OR NA+-CA2+ EXCHANGER MAY BE USED TO EXPLAIN THE PRESENT RESULTS. DR. YAMOAH WAS SUPPORTED BY AN MBL GRASS FELLOWSHIP AS A GUEST OF THE NVPF.